The UARC 146.62
Synchronous/Voting Repeater System
Peak and Scott's Hill
The UARC 146.62 repeater system presently consists of two
sites - the Farnsworth Peak
and Scott's Hill
These are linked together full-time and function as a single
wide-coverage repeater - see the Coverage
Map to find out where!
The Scott's Hill site,
looking toward the north and east with the lights of Park
City visible in the background. This site provides
coverage east of the Wasatch into the Park City and Heber
areas as well as into southwest Wyoming - not to mention
helping to fill in "holes" of coverage elsewhere!
Click on the picture for a larger version.
What is a Synchronous/Voting repeater system?
Although used frequently in the commercial radio world,
synchronous and/or voting repeaters aren't too common in
Amateur Radio use. Knowing that, however, you may
still be wondering what, exactly, a "synchronous" (also
called "simulcasting") or "voting" repeater system
If you have used 2 meter or
70cm FM very much at all you'll already be familiar with
what happens if two transmitters key up on the same
frequency: Often called "doubling",
one typically hears a bit of a squeal and a mish-mash of
noises as the signals from both stations obliterate each
other, making both of them uncopyable if they are of
similar signal strength.
You may have also noticed
something else that occasionally happens: One
station will "win" the double by overriding the
other. In these situations you may not even have
noticed that two people have keyed up at the same time
until someone unkeys and you hear the "other" person that
had been transmitting underneath. In these cases you
witnessed the "capture
effect" - that is, the ability of a strong FM
signal to override the other with little evidence of the
In the first case where
there was no clear winner - only a mish-mash of noises -
the two signals were probably of roughly equal strength at
the receiver (either yours, or the repeater's!) but if one
signal is much stronger than the other, the strongest one
comes through quite clearly with, perhaps, only a little
bit of noise in the background being the only evidence
that there was someone else transmitting at the same time.
Why, then, would one
intentionally put two transmitters on the same
frequency if they are going to clobber each other?
Again, it comes back down to
that "capture effect" - but with a twist: If the two
transmitters are very close to the same
frequency and they carry the same audio, it
turns out that they do not obliterate each other as they
would otherwise. Having two transmitters on the same
(or very close to the same) frequency is often referred to
as having synchronous transmitters - and it
is also called simulcasting.
In reality, the two
transmitters do not need to be on exactly the same
frequency, but just "very close" (within a few 10's
of Hertz) to each other If this is the case then one
will typically hear one transmitter OR the other
at any given instant with "destructive interference"
occurring only when the signals from both transmitters are
of nearly identical signal strength - but even then, the
two signals probably wouldn't obliterate each other, but
more-likely cause a little bit of distortion - but not so
much that intelligibility was compromised.
You may be familiar with the
fact that in some cases you can move your Handie-Talkie's
antenna just a few inches to find a "hot spot" and
considering that, you can already appreciate how hard it
might be to find a spot where two signals are of precisely
the same strength and if you are in a moving vehicle, you
probably won't notice that you are hearing two, identical
transmitters at all! If you are mobile in an area
where the signals between the two transmitters are
approximately equal overall, you probably won't notice any
evidence of the two separate transmitters - aside from a
bit of "picket
fencing" as your receiver randomly hears one
transmitter or the other. In testing, we have found
that offsetting the two transmitters by somewhere in the
either 1-4 Hz or 35-60 Hz area provide the most
aesthetically pleasing effect in those areas where the
signal strengths of the two transmitters are precisely
equal: For this reason, the term "synchronous" may
not be completely accurate.
Why have multiple
So, what's the advantage of
having two transmitters on the same frequency, then?
If you have these two transmitters at different sites -
especially ones that have minimum to moderate overlapping
coverage - you can then use just one
frequency to cover a larger geographical area. Not
only does this conserve frequency spectrum by using just a
single frequency, but it also makes using the system much
easier as you don't have to keep switching
from one repeater's frequency to another as you move
through their coverage areas - as you would had you been
trying to use different-frequency repeaters that were
There's another benefit of
having two transmitters on the same frequency and that's
the fact that the total coverage is greater than the sum
of its parts! In other words, in those "fringe"
areas where you might get little bits of signals from
either transmitter, having multiple transmitters at
different sites on the same frequency helps "fill in"
those tiny gaps in overage where one would otherwise have
to constantly switch between different repeaters to find
the best coverage as you would in a conventional linked
system where the transmitters were on different
For a set of maps
showing the predicted coverage of the Farnsworth
Peak, Scott's Hill and the combined transmitters,
visit the 146.620 Coverage
The receive site at
Farnsworth Peak. This site provides coverage along the
west side of the Wasatch Front, to the west past the Nevada
border, to the north into Idaho, and beyond Utah county to
Click on the picture for a larger version.
If you have wide-area
coverage with multiple transmitters, what good is it if
the users can't get back into the system everywhere that
it covers? That's where having "diversity
reception" - multiple receivers tuned to the
same receive frequency - comes into play.
As you might imagine, two
receivers located at two different sites won't hear the
same mobile transmitter equally: Usually, the one
closest to the transmitting station will get the best
signal while the more-distant one will be more-likely to
have the worse signal. Again, if you've used FM very
much, you'll know that this isn't always the case as local
obstructions (buildings, hills, mountains, etc.) will
sometimes cause blockage of the more-local repeater while
you can still get into the more-distant repeater that
might lie in a direction with fewer obstructions.
In the case of a voting
system, all the audio from the different
receivers is sent to one place and there the voting
controller analyzes all of the signals from every receiver
in the system and then picks the best one
to be transmitted by all of the sites.
How does it "know" which is
the best signal?
Using FM, you may be aware
of something else that happens as signals get weak:
The audio doesn't get softer as the signal gets
weaker, rather it gets noisier until the audio is lost
completely - in the noise: It is this same "noise"
that your receiver uses for the radio's squelch - and by
the time it gets too noisy, your squelch has
probably closed and you don't hear anything! It
makes sense, then, that if you have two signals from two
different receivers that you see which of the two is noisiest:
Knowing that, you can decide not to use the
noisiest signal(s) and pick the "cleanest" one instead!
Imagine, then, that you have
several receivers listening to the same mobile signal as
it drives around amongst buildings, trees, hills and
mountains. Sometimes it may be getting into one site
better than the other, but other times it may be the other
way around. By having a voting system constantly
looking at the signals from all of the receivers we can
always take the best signal at any instant from the
receiver that has it!
Just as with synchronous
transmitters, the coverage of the entire receive system is
greater than the sum of its parts! If you are in a
location that isn't covered well by either system alone,
there's a better chance that with multiple receivers
hearing you you'll have a good signal into at least one
of them at any given instant, and with the system
automatically looking for the "best" signal, it will be that
one that will be heard by everyone listening. Unlike
a linked repeater system with different repeaters on
different frequencies, you don't have to worry if you were
lucky enough to happen to choose the "best" receiver for
How can I tell what repeater site I'm hearing?
Because all receivers and
transmitters in the 146.620 linked system are operating on
the same frequency, you may wonder how you can tell what
site you or someone else is getting into and/or which
transmitter you are hearing. In order to be able to
tell which receiver is hearing you and which transmitter
you are hearing, some "beeps" have been added. If
you listen carefully, you can determine both which
transmitter you are hearing and where the
last person that transmitted was being received.
Where the beeps
transmissions from Scott's Hill have a "transmitter
beep" that occurs just before the
repeater itself un-keys, but the transmissions from
Farnsworth do not
have this beep.
- Those signals that were received at Scott's
Hill have a beep added to them just after the
user on Scott's unkeys - but a half-second before the repeater
itself drops: Signals going into Farnsworth don't have a
beep added to them.
Unless you have
a trained ear, you may have a bit of difficulty trying
to figure out exactly when each beep occurred, so here
are some simple rules:
No beeps at all -
The different sounds and
beeps heard on the 146.620 repeater system.
Top Left: Hearing someone getting into
Farnsworth, via Farnsworth. Notice that there are NO
Top Right: Someone getting into Scotts, but YOU
are listening to Farnsworth. Notice the beep immediately
after the first squelch burst.
Bottom Left: A user getting into Farnsworth who
is being heard via Scotts. Notice that in this case
the beep happens immediately before the second
Bottom Right: The squelch tail from a user
getting into Scotts who is also being heard via
Scotts. It is only in this case that you will
hear two beeps - like a cuckoo-clock!
Left-click on an image to "hear" what that squelch
tail sounds like, or right-click to view a larger version
of each image.
- If you are hearing a
station with no beeps at the end of the
transmission at all, then you can be sure that you are
hearing someone who:
- Is being received
at Farnsworth Peak, and
- That you
are hearing Farnsworth Peak.
You will likely
hear NO beeps at all if you are along the
Wasatch front (Salt Lake, Ogden or Provo) and talking to
someone else who is also operating from a location along
the Wasatch front.
You are hearing
TWO beeps - audio
You will likely hear two
beeps if you are in a location east of the Wasatch and
you unkey, or if you are talking to someone else who is
also east of the Wasatch.
- If you are hearing two
beeps that sound very much like a cuckcoo
clock - one immediately after the station transmitting
into the repeater unkeys and other one just before the
transmitter drops, then:
- You hearing a
station that is getting into Scott's Hill -
which accounts for the first
of the two beeps.
- Your receiver
is also hearing the Scott's Hill's transmitter -
which accounts for the second
of the two beeps.
You will only
hear two beeps if you are in a location where you can hear the
Scott's Hill transmitter instead of Farnsworth, and this will
probably happen only if you are somewhere to the east of the
If you are
hearing ONE beep:
Unfortunately, this can be
confusing at first - at least until you know exactly what
to listen for.
- Hearing someone
getting into Scott's - but YOU are listening to
- Again, if you hear a
beep just after the other person unkeys
(as in, just after the "ker" in "kerchunk") or if you
are hearing someone who is getting into Scott's - audio clip.
For those who are musically-inclined, the "Scotts
Receiver" beep is close to being a "G".
- If you are in Salt Lake, Provo or Ogden or anywhere west
of the Wasatch mountains: If you hear a beep, you
are hearing the Farnsworth transmitter and the beep indicates
that you are listening to someone who is getting into Scott's.
- You are hearing
Scott's, but listening to someone who is getting into
- If you hear a beep just
before the repeater drops (just before the
"chunk" in "kerchunk") you are hearing a "transmitter"
beep - audio clip.
The "Scotts Transmitter" beep's pitch is close to a
- If you are east of the Wasatch mountains, such as
past Parley's Canyon on I-80, or in the Park City or Heber
areas: In these areas, you will be hearing the
Scott's Hill transmitter rather than Farnsworth - but you are
hearing someone who is getting into Farnsworth. In this
case the beep you are hearing is from the Scott's Hill
Different numbers of beeps:
As you might guess, if you are leaving an area that is covered by
one transmitter and going into an area covered by another - or if
you are listening to someone who is mobile and moving between
coverage areas - you can expect to hear different numbers of beeps
as you (or the person using the repeater) goes in and out of the
various coverage areas. This shouldn't be too surprising,
however, as one area is covered better by one site than another!
Sometimes, a person is in a location where they can get into both
Farnsworth and Scott's equally well. In this case, the voter
will randomly choose which receiver (the one from Scott's or the one
from Farnsworth) to use. Since both are equally good signals,
there's no real way to tell which one is "better" so it's the luck
of the draw! It is often the case that people living on the
west side of the Salt Lake valley with good antennas will randomly
get into either Scott's or Farnsworth and no those people, you'll
sometimes hear beep - but sometimes not!
Technical stuff: How the system works
If you are are a technical
person and are wondering how this system works, read
on! If you aren't technical - but are still interested
- read on, anyway.
Synchronized Transmitters (a.k.a.
Technically, the transmitters
aren't actually synchronized to each other, but are held to
the intended transmit frequency with high accuracy -
typically being within 1-2 Hz of their intended
frequency. To do this, 10 MHz oven-controlled
crystal oscillators (OCXO's) are used at each
transmitter to provide a highly-accurate (approximately
10E-8) frequency reference. In the years since the
system was installed, the transmit frequencies of the two
sites have been observed to stay within 2 Hz or so of where
they were set: We haven't checked to see if both sites
are staying on their respective frequencies or if they just
happen to be drifting together, in the same direction - but
it doesn't matter, really!
The "guts" of the Disciplined
Oscillator. This unit locks a standard GE "EC" ICOM to
a 10 MHz OCXO to provide precise frequency control.
There are two such units - one each at Scott's and
Click on the picture for a larger version.
Setting frequency offsets:
Even though the transmit frequencies can be set to within
1-2 Hz of each other, one may not wish to do this,
particularly for those areas in which signals from both
transmitters may be heard.
Soon after installation, we heard from Brett, W7DBA, who
lives in Huntsville - a community east-ish of Ogden and
behind the mountains and it turns out that he doesn't hear either
Scott's Hill or Farnsworth very well, but hears both equally
poorly. As luck would have it, propagation has often caused
both signals to be of almost identical signal strength at his house
when received on an omnidirectional antenna! Needless to say,
this particular situation is unlikely to occur, but Murphy's Law
would seem to make it inevitable for someone!
As the opportunity arose, we worked with him to find a frequency
offset that provided the "least annoying" effects, and here is what
Based on these observations, we
left the frequency offset in the 50-60 Hz range for a while
before changing it back to something in the 2 Hz range after
further field observations.
- Offsets of <1-4 Hz: A slow "swishing" sound,
but when the propagation effects caused the signal strengths of
the two transmitters to be precisely equal to each
other, the squelch would close and parts of words/syllables
would be lost. Most of the time, it wasn't particularly
bad or annoying as it wasn't too often that the signal strengths
were close enough to each other than a complete signal
cancellation would occur.
- Offsets of 4-10 Hz: Ugh! This caused words
to be chopped up - like trying to talk through a slowly rotating
- Offsets of 10-30 Hz: While word intelligibility
was improved, this was considered to be pretty annoying -
especially at the low end of the range below 20 Hz! He
likened the effect to an engine running nearby, with the words
chopped up. Even though it was somewhat more
intelligible than the 1-2 Hz offset range, listening to it was
- Offsets of 40-60 Hz: Although there was a bit of
a "buzz" in the background, it was quite copyable when the two
signals were of nearly-equal strength, it wasn't "bad" to listen
to. Although not as aesthetically pleasing as the 1-2Hz
offset rate, at least audio didn't get holes punched in it -
causing the loss of syllables - if the two signal strengths were
In addition to adjusting the frequency offset, it is
important that the audio phase of the transmitters be set
equally - that is, all transmitters' deviations should go up
and down at the same time. If this isn't done,
addition distortion can occur on the overlap areas.
Another factor that can affect how overlapping transmitters
affect each other is the time-of-flight delay. Since
light travels at a finite speed, the signal from the nearby
transmitter will reach you before the signal from the
more-distant one will. More significant, however, is
the fact that the "slave" transmitter not only has to send
its audio to the master site (on Farnsworth) but then
Farnsworth has to send it back again. Not only is this
extra "round trip" adding a bit of extra delay, but also do
the transmitters, receivers in the link to and from the
master site. As it turns out, matching this delay is
less-important than many make it out to be - particularly if
only radio links are used. It is far more
important to make sure that the phases of the audio match
overall (particularly in the low-to-mid speech range) and
that the frequency offsets are adjusted to reasonable
How it works:
For transmitting, GE Mastr II exciters are used at each
site, but instead of using standard channel elements (called
"ICOM"s by GE) there are modified channel elements that plug
into the standard, unmodified GE exciter that accept
oscillator-frequency inputs from an external source - a
device called a "Disciplined Oscillator." This
Disciplined Oscillator module uses a PIC-based synthesizer
to precisely lock a standard, unmodified GE
"EC" type channel element to the crystal frequency - which
is, in this case, 1/12th of the transmit frequency, or
12.218333 MHz. Using DDS
techniques, the crystal frequency can be
adjusted and the ultimate transmit frequency can be tweaked
in steps of approximately 2 mHz (yes, that's
millihertz and NOT Megahertz!) at the
VHF transmit frequency, so the ultimate accuracy
of the transmit frequency is essentially that of the 10 MHz
OCXO. The output of the "EC" ICOM in the Disciplined
Oscillator is then buffered and fed to the exciter via
coaxial cable to the modified channel element.
Why use the GE Mastr II exciters? For one thing, they
are readily available on the surplus market. Another
important point is that the GE radios are phase-modulated
instead of being frequency-modulated.
Why is this important? With phase-modulated
transmitters, the actual frequency source of the transmitter
(in this case, that being produced by the transmitter's
channel element) is un-modulated and simply
needs to be held to the correct frequency.
Practically speaking, one could use a frequency-modulated,
synthesized transmitter and simply make sure that the
reference frequency of each transmitter is held to very
tight standards, but those who have carefully observed such
transmitters have noted that the actual frequency tends to
wander around the intended frequency and only on "average"
does it tend to stay close to intended frequency. It
is these low-frequency variations that are most-critical in
a synchronized-transmitter system: Modulated PLL
(Phase-Locked Loop) transmitters tend to wander around 10's
of Hz (or more!) with modulation as the PLL tries to keep
the oscillator on frequency - despite the modulation put on
it for the express purpose of knocking it
off-frequency! It is these slight differences (10's of
Hz to 100's of Hz) in transmit frequency that tend to be
most-noticed in the overlap areas between transmitter sites.
A phase-modulated transmitter, on the other hand, has a
rock-solid oscillator frequency that never changes. It
is also the intrinsic nature of the phase modulator to
resist low-frequency modulation as it is, in fact, rather
difficult to achieve much modulation at low audio
frequencies (such as those used for subaudible
tone encoding) with a phase modulator. By
virtue of this fact, the phase-modulated transmitters are
arguably rather better-suited for synchronous operation in a
communications radio system such as an amateur repeater!
For Scott's Hill a minimally-modified GE radio is used for
transmitting: The only modification required is that
to bring in the signal from the Disciplined Oscillator from
the outside requiring the installation of a connector on the
transmitter's case. The actual transmitter itself is
stock, with the modified "EC" channel element being plugged
in where a standard channel element would go! In fact,
should the Disciplined Oscillator fail, one could simply
unplug the "EC" element from it, put it in the exciter, and
then set it to frequency as one would on a "normal" repeater
as it would still get its compensation voltage from the
receiver's channel element: In this configuration the
transmit frequency would not be held to such great accuracy,
but it would still be perfectly functional.
For Farnsworth Peak, the original, circa 1978 2-watt exciter
was replaced with new exciter based on an unmodified GE
Mastr II exciter. The exciter board was mounted in a
shielded enclosure along with a regulated 10 volt supply, a
simple transmitter keying circuit and a voltage reference
for the temperature compensation line to allow possible
emergency use with the un-locked "EC" element. As with
the Scott's Hill transmitter, the signal from the
Disciplined Oscillator is brought in via a modified "EC'
channel element but it, too, could also operate at reduced
frequency accuracy by installing the channel element from
the Disciplined Oscillator. The output of the exciter
is boosted to the 2-3 watt level using an external amplifier
and that is used to drive the same 100 watt external power
amplifier that had originally been driven by the old
The Disciplined Oscillator module has another
function: To control the local VHF transmitter and the
UHF link transmitter. Since the Scott's Hill
site is normally functioning as a dual cross-band repeater,
there needs to be a means to provide transmitter control,
provide a legal ID on the UHF link transmitter, and to
insert the 3.2 kHz "squelch tone" on the UHF link to
Farnsworth as noted below. For the VHF transmitter,
the Disciplined Oscillator inserts a "transmit beep" to
identify to the users that they are hearing the Scott's
transmitter and not Farnsworth (see above.)
The Disciplined Oscillator (as well as the Voting
Controller) have RS-485 multi-drop interfaces that allow
them to be connected on a common bus, providing a means of
remotely controlling and configuring them.
Getting the received signals from Scott's Hill to
To allow the use of multiple
receivers for "diversity
reception", the audio and COS (Carrier
Operated Squelch) information from the Scott's
Hill receiver is shipped to Farnsworth Peak via a 70cm
link. At Scott's, the audio from the 2 meter receiver
is placed on the 70cm link transmitter along with a 3.2 kHz
"squelch tone" that is used to indicate the COS state of the
Scott's receiver: When the 2 meter squelch is open on
Scott's, the UHF transmitter is keyed up and the audio from
the VHF receiver is relayed on the link, but the instant the
2 meter squelch on Scott's closes the receiver's audio is
muted and the 3.2 kHz tone is activated and transmitted for
the duration of the "hang time" of the UHF link transmitter.
The front panel of the
8-channel voting controller. Status LEDs indicate
current and past receiver/voting activity. The UHF
link radio (to/from Scott's) is partly visible above.
Click on the picture for a larger version.
At Farnsworth the COS signal from the UHF link receiver and
the presence of the 3.2 kHz tone together form the COS
signal used to feed the voting controller: If the COS
drops or if a 3.2 kHz tone is detected, the
voting controller gets a "squelch closed" indication from
Scott's, but if the UHF receiver's COS is active without
a 3.2 kHz tone being present, the signal is considered
Why is there a 3.2 kHz tone at all? In theory, the VHF
receiver's COS could have been used to key the UHF link
transmitter and the UHF link receiver's COS could have thus
signaled the VHF COS state at Scott's, but this scheme would
inevitably introduce an extra burst of squelch noise as the
UHF receiver's squelch opened and closed on "fluttery"
signals. By having the 3.2 kHz squelch tone signaling
the closing the the squelch at Scott's these extra "squelch
noise" bursts are eliminated. At Farnsworth there is a
detector specifically for the 3.2 kHz tone, but it does not
respond instantly so a 3.2 kHz notch filter in the voted
audio prevents users from ever hearing that tone at all.
The 3.2 kHz tone also serves another purpose. The
voter itself determines the signal quality by comparing the
amount of energy of each receiver channel after it passes
through a 3 kHz, 3-pole Butterworth high-pass filter.
Actually, this 3 kHz filter doesn't resemble much of a
"brick wall" as it still allows quite a bit of audio energy
through down to 2 kHz or so, but it is these "higher" audio
frequencies that tend to "noise up" first as signals get
weaker and it is this "noise plus audio" that is actually
measured: By seeing which of the channels has the most
"noise plus audio" one can determine which signal is, in
fact, the noisiest overall. Since the 3.2 kHz squelch
tone is quite strong, it will be "seen" by the voter as
"noise" and since that tone only appears at the instant that
the squelch from Scott's closes, it further reinforces the
fact that the signal from Scott's at that moment is "bad"
and should not "win" the vote.
Using the "local" receiver at Farnsworth:
For the "local" signal from Farnsworth - that is, the
"original" receiver - we had the luxury of having direct
access to the COS and audio lines after modifying the
existing controller. Almost from the beginning, the
Farnsworth Peak repeater has had a remote-controlled squelch
to allow adjustments without having to visit the the
mountaintop - a near-impossibility in the dead of
winter! Because the repeater uses a receiver located
at a remote site several hundred feet away - and the
tendency of a high mountain to accumulate lighting, all
signals had to be transformer-coupled - including a
PWM-encoded squelch signal.
With the squelch circuity entangled within the original
controller, modifications were made to break out the
receiver audio and the decoded COS line. In addition
to these two lines, this modification also made available a
"COS Input" and "Receive Audio Input" line by the simple
expedient of interrupting those lines internally and routing
them externally, allowing us to insert the Voting controller
into the circuit: This had the effect of allowing the
Voting controller to be "seen" by the original repeater
controller as just a single receiver. For diagnostic
and "backup" purposes there's even a "voter bypass" switch
that returns the controller to its original configuration,
allowing it to look at the Farnsworth receiver only.
The Voting controller, like the Disciplined Oscillator, is
custom-made and it has provisions to allow up to 8 different
receivers to be used. To accommodate varying levels of
audio and the expected different amounts of audio
"coloration" - both of which could affect the ability of the
controller to vote equitably - a number of different
parameters are provided to adjust channel gain as well as
the "noise detector" gain. With the proper adjustments
the voter can be "tweaked" so that equally-noisy signals
from the two different receivers - the one from Scott's via
the UHF link and the one from the local Farnsworth VHF
receiver - can be also voted equally, or even adjusted to
slightly favor one site over another.
Even with the adjustments available on the voter, a 3.5 kHz
lowpass filter was added at the input of the local receiver
audio at Farnsworth. This was done because, unlike the
audio from Scott's, this is "local" audio, fed by copper
wire from the receiver and as such it contains more
high-frequency energy in its audio than does the audio from
Scott's, which loses some of its high-frequency content
because of its passing through the link transmitter and
receiver. With that "extra" high frequency energy
present from the Farnsworth "local" receiver the voter would
tend to see more of what it considered "noise" on the audio
of those signals being received at Farnsworth than it would
on a signal of equal-quieting being received at Scott's and
passed over the link and therefore, it would tend to deem
signals received at Farnsworth to be "worse" than those
received from Scott's even if they were equal. The
addition of this lowpass filter is generally "invisible" to
the user, but not to the voter!
An additional feature provided by the voting controller is
the ability to insert a "Squelch Beep" on a received signal
after its COS drops. At the moment this feature is
used only to indicate signals that originate from Scott's
The link back to Scott's:
The output of the original Farnsworth controller feeds into
the Disciplined Oscillator module which, in addition to
frequency control, has another function: To provide
direct control of the local VHF transmitter and the UHF link
transmitter. In the case of Farnsworth, the
Disciplined Oscillator uses the PTT signal from the old
controller to "know" when to key both the VHF and UHF
transmitters. Additionally, the two audio inputs of
the Disciplined Oscillator are fed from one source - the
output of the original repeater controller.
While the Disciplined
Oscillator inserts a "transmitter beep" on the VHF
transmissions from Scott's, this feature is not used on
Farnsworth as no beeps were required and the audio is passed
straight through the the VHF exciter with this unit adding
some extra hang-time. For the UHF transmissions to
Scott's, however, as soon as the PTT signal from old
controller drops, a 3.2 kHz "squelch tone" is activated,
signaling to Scott's Hill that its hang time is to
begin. In this way, the extra burst of noise from the
UHF link's squelch tail is suppressed and the hang-time of
both the Scott's Hill and Farnsworth Peak VHF transmitters
may be precisely synchronized to "drop" at the same time.
Wider-angle (top) and a
zoomed-in view of Scott's Hill, as seen from Farnsworth
Peak. Both of these pictures have been "adjusted" to
compensate for some of the haze in the air at the time the
picture was taken.
Click here for an annotated
version of the lower picture that points to the
precise location of Scott's Hill.
Click on the picture for a larger version.
"YAFB" (or, "Yet
Another Fine Box"):
Because the new devices have RS-485 interfaces, it is
possible to interconnect them and provide control via
one interface. One option of this "command and
control" is to have a device controllable via the
receiver using DTMF, returning information to the
These boxes are
present at both Farnsworth and Scotts and allow the
control operator to remotely read and modify
parameters as well as reporting the status of various
devices, and it can do so in either Morse or ASCII
AFSK (e.g. RTTY) allowing either "human" interfacing
or a computer running an RTTY program.
Hill remote squelch:
While we were
working frantically to complete the installation of
the gear on Farnsworth before winter weather moved in,
we became painfully aware of a problem that we were
expecting to happen - but hoped would not:
Scott's Hill's 2-meter receiver started to blow
The reason for
this wasn't clear, but it was probably due to a
transmitter from another service radiating some
garbage under certain conditions, but regardless of
the cause, the result was the same: The '62
repeater became unlistenable and we had to shut off
the Scott's link.
Earlier in its
history this lesson had been learn on Farnsworth,
necessitating the use of a remotely-controlled squelch
control which has been occasionally invaluable over
the years as it saved having to take a trip to the
mountaintop (or attempting to convince one of the
people that work up there) just to turn a knob.
In addition to having remote-control capability, it
also allowed one to precisely set the squelch to the
same setting repeatedly - something not easy to do
with a squelch knob!
installation work at Farnsworth we dashed over to
Scott's in the early hours of the morning - chaining
up the tires and braving the hazardous road, just
hours before a large storm front moved in - just to
turn a squelch control a fraction of a turn!
Making a guess, we turned it a bit farther than we
really wanted to - but we decided that we'd rather
have a somewhat "deaf" receiver than one that kept
In 2010 - during
the same trip as the installation of the "YAFB" (see
above) we finally installed a
remotely-controlled squelch on the 2-meter Scott's
receiver. This is essentially a
computer-controlled adjustable-gain amplifier that is
connected inline with the original squelch
control. The modification to the GE Mastr II
receiver was simple: The addition of a 1/4"
switching-type audio plug. When the remote
squelch was unplugged, the receiver's original squelch
control functioned exactly as originally designed!
With this new
box we can now set and read the 2-meter squelch on
Scott's remotely - and this box also provides a few
additional useful functions: It can read the
power supply voltage - which gives us one way of
telling if the Scott's site is on AC power or battery
backup - as well as the temperature inside the
building on Scott's Hill and recording the minimum and
maximum of each since the last "Min/Max reset."
The "Scott's Hill" controller:
There is yet another box at Scott's Hill - a repeater
controller. This is a modified NHRC-4 controller and its
function is simple: To make the system legal.
Normally, Scott's Hill operates as a dual cross-band
repeater: 2-meter signals received at Scott's are relayed to
Farnsworth on UHF and the UHF signals from Farnsworth are
retransmitted from Scott's on 2 meters. The Disciplined
Oscillator provides an ID for the UHF link to Farnsworth while the
normal repeater ID from Farnsworth on the UHF link and is thus
repeated, identifying both it and the VHF transmissions from
Scott's. In this mode the NHRC-4 controller does absolutely
nothing other than "listen" for commands.
What if we needed to turn Scott's off for some reason? To
do this the NHRC-4 controller can disable the transmitter control
provided by the Disciplined Oscillator and put the repeater in
"Standalone" mode. In this mode the Scott's Hill site can
function as an independent repeater, apart from Farnsworth Peak
and once so-configured, one may disable the on-site transmitters
Comment: At the moment the controller at Scott's
Hill is running the original NHRC-4 software. Eventually,
this software will be replaced with a custom-written version as
there are a number of things that we would like to have the
controller do, but it can't, owing to the fact that we are using
the controller in a way that the original designers didn't
envision. For the time-being, however, it serves its main
purpose of being able to remotely turn the Scott's repeater on
Q&A about the system:
- Q: Are your transmitters GPS-locked?
- A: No, high-stability 10 MHz Oven-Controlled
Crystal Oscillators (OCXO's) are used instead. These OCXO
are medium-grade devices extracted from discarded (circa 1990)
satellite gear, so their crystals are well aged - plus we have a
large quantity of them onhand. These devices were tested
and then modified (with the addition of a single 10k resistor
between the oven's internal +5 volt supply and its "Vtune" pin)
and none of the devices that we selected at random displayed any
problems. Between the two sites (Scott's Hill and
Farnsworth Peak) the frequency difference between the
two sites seems to stay within 1-2 Hz of the target over time
and over a variety of temperature ranges. If we were to
use "better" oscillators, we could, no doubt, hold the
tolerances even better.
- Q: Why didn't you use GPS-based references or
Rubidium references to hold the frequency better?
- A: Expense, power consumption, reliability and
size - in that order.
- Expense: Good-quality disciplined GPS receiver modules
(such as the HP Z-3801) are scarce these days and rather
expensive. Second-hand Rubidium references may be
obtained for under $100 and provide more than adequate
frequency stability and accuracy.
- Another concern is power consumption. While not a big
issue at Farnsworth, Scott's - which is in a remote location -
can suffer from power outages lasting days. Adding
another 10-50 watts of power consumption with either a GPS or
Rubidium could make a very serious dent in the longevity of
the system when commercial power has failed.
- While there are other devices around that offer good
performance for far cheaper, reports from those who have used
them at repeater sites have indicated that they aren't
particularly reliable for one reason or another.
- One difficulty has to do with the GPS receiver system
reliability and one common failure mode seems to be the
outside antenna - one of the more vulnerable components in
the system as it is subject to weather degradation
(especially if used at a site with severe conditions) as
well as the potential to suffer from interference - either
from harmonics/mixing products that just happen to land
at/near the GPS frequency or from simple overload of the
antenna. This latter problem can be mitigated through
the use of a higher-grade antenna - one with built-in
filtering prior to the LNA.
- How about a Rubidium reference? While, for our
purposes these would be as accurate as a GPS receiver, these
devices contain a rubidium lamp with a definite
lifetime. Since the only affordable rubidium boxes are
usually "pulls" from commercial service, they already have a
lot of "miles" on them (they are often 8-10 years old or
more!) and it was feared that these could be a source of
trouble on sites that had no ready access for most of the
- Finally, size is somewhat of an issue at Farnsworth.
While we could probably do it, finding space for yet another
box in the rack would be a challenge.
- Q: How do you match the audio delay between the
two transmitter sites?
- A: We don't bother, really. The only thing
that we check is the phase to make sure that for lower audio
frequencies (say, 1 kHz) that the two transmitters are more
in-phase than out of phase. While we thought it might be a
concern originally, we decided to hold off on that particular
design aspect unless/until we determined it to be a problem via
field observation - and it just isn't! One fact to our
advantage is that the Farnsworth and Scott's sites have little
or no overlap in populated areas, so only a very small number of
people would ever experience the two transmitters' signals of
equal strength at their home stations: All others would
likely be mobile through areas of overlap and one transmitter or
another would be captured at any given instant. In
testing, we deliberately sought out areas with severe overlap -
that is, the heterodyne caused complete nulling (to the
receiver's noise floor, at least) of the two carriers and ran
some tests. The difference between the two transmitters in
and out of phase with each other was surprisingly small, but we
selected the "in-phase" setting as there was an audible
reduction in distortion. If we added another transmitter
with significant overlap we might have to revisit this issue,
but until then...
- Q: How did you select the offset frequency?
- A: By determining what was most aesthetically
pleasing to the listener. Listening in an area with
significant overlap the frequency offset between the two sites
was adjusted. The worst settings were those around 2-10 Hz
as this made the audio sound like one was listening through the
blades of a fan. A very low offset (2 Hz or so) and a
rather high offset (50-70 Hz) were both considered to be
acceptable even though they caused audible artifacts, but the
general preference was with the low offset - and that's where
we've been running it. Having designed and built a
GPS-based synchronized system in the past in which all
transmitters were locked to exactly the same frequency, I
observed that at UHF frequencies (approx. 450 MHz) the phases of
the RF carriers of the individual transmitters seemed to drift
about 45 degrees with respect to each other over the course of
several hours. The result of this was that the "standing
nulls" would drift around over time and that in an area with
severe overlap, a stationary radio (in a vehicle or a
handie-talking sitting on a table) may or may not fall into one
of those nulls at random. Having a small amount of offset
prevents this, allowing the user to easily find a spot (by
moving the antenna even slightly) to favor one transmitter or
- Q: Why didn't you use digital linking for the
- A: KISS - that is, Keep It Simple
Stupid! In many ways this system is no more complicated
than a run-of-the-mill linked repeater system, using
conventional and ready-available gear to do the majority of the
work. As such, we have readily-available spares (you can
get them on Ebay!) for the majority of the gear that we use and
we can use normal test equipment (or often just an HT) to
diagnose and test the various pieces! About the only
"special" gear is the Voter (we could have used, say, a GE Voter
shelf it we'd wanted...) and the frequency-control system - and
for that we might have been able to get away with installing our
crystals in re-purposed crystal ovens to obtain adequate
frequency stability - and then tied the entire thing together
with an off-the-shelf controller. As it turns out, we did
it the way that we did it and have been gratified that there
have been very few problems!
- Q: What problems have you
- A: As complicated as things are, the number of
problems have been surprisingly small and many of them have been
unnoticed by the users. From worst to least, here are a
few of the problems that we have had over the year or so that
the system has been in operation:
- UHF Link receiver. Since we dragged one of our
UHF link receivers about 30 MHz below its original design
range, we later discovered that one of its tuning components
is excessively temperature-sensitive. A simple
mechanical modification in the helical resonator fixed this.
- Timing issues. In 2009, when we put the Scott's
Site online, we made a rough guess as to its timing
parameters. We also discovered, at that time, a "glitch"
in the way the UHF link squelch system worked and this
required that we extend the squelch tail of the system from
its normal 600 milliseconds to about 1.5 seconds.
Because Scott's Hill became inaccessible (due to snow) soon
after installation, it stayed this way until July 2010 when
the road reopened and modifications/adjustments could be
made. Now, the two repeaters key and unkey within a few
10's of milliseconds of each other.
- Minor voter issues. While it's pretty easy for
the human ear (and the attached brain) to determine the
"quieting" of a signal, it's a bit more difficult to do this
automatically. The voter system used here compares the
"high frequency" (above 2.5-3 kHz or so) content of the
receive audio sources and picks the "quietest" one.
Early on, there were some issues with the voter - notably the
gain of the "noise detector" - which had to be increased
significantly. Also added to the voter's software were
additional parameters to allow tweaking of the individual
voter channels to adjust gain and how they responded to
rapidly-changing signals. Now, the voter works pretty
well - although it still has a difficult time distinguishing
between signals that are completely full-quieting and those
with just a little bit of hiss. (Then again, almost all
audio-based voters have this problem!)
- Q: What does the future hold for this system?
- A: That's hard to answer and other than doing
some fine tweaks on the existing gear, there are no large plans
for immediate changes - but there are plenty of "hooks" in the
system to allow expansion. For example, only 3 of the 8
voter inputs are being used (one of them being used for
telemetry purposes) and it is practical to add any number of
extra VHF transmitters to the system - provided that they can
"hear" the UHF transmitter on Farnsworth - something that could
be used to further-expand coverage and/or allow the addition of
additional low-power transmitters to "fill in" gaps in coverage.
This page last updated on 20111W107
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